1. Field of the Invention
This invention relates to a unit fuel injector for use in an internal combustion engine such as a diesel engine.
2. Prior Art
A unit fuel injector comprises a pump mechanism and an injection nozzle mechanism both of which are incorporated in an injector body, the unit fuel injector being mounted on an engine. The pump mechanism includes a cylinder bore formed in the body, and a plunger received in the cylinder bore for reciprocal movement there along. A pump chamber is defined by the cylinder bore and the plunger. The volume of the pump chamber is reduced during an advance or pump stroke of the plunger, and is increased during a return or suction stroke of the plunger. The injection nozzle mechanism includes an injection port in communication with the pump chamber, and a valve disposed between the pump and the injection port. When the pressure of fuel within the pump chamber is increased to a high level during the pump stroke of the plunger, the valve is opened to inject the fuel from the injection port. The unit fuel injector is also provided with a fuel relief control mechanism by which the relief of the fuel pressure within the pump chamber is controlled during the pump stroke of the plunger so as to control the timing of starting the fuel injection and the timing of terminating the fuel injection. As one example of such fuel relief control mechanism, an electromagnetic valve mechanism has been proposed.
U.S. Pat. No. 4,674,461 discloses a unit fuel injector provided with such an electromagnetic valve mechanism. More specifically, there is provided a relief passage communicating at one end with a pump chamber, the other end of the relief passage being opened and closed by a needle-type valve member of an electromagnetic valve. In this conventional unit fuel injector, during the time when the fuel injection is being effected, with the relief passage closed, the valve member is subjected to a very high fuel pressure. Therefore, to cope with such a high fuel pressure, an associated solenoid is required to produce a sufficiently great force for holding the valve member in its closed position.
U.S. Pat. Nos. 4,392,612, 4,463,900, 4,485,969, 4,527,737 and 4,618,095 also describe unit fuel injectors incorporating an electromagnetic valve mechanism. More specifically, such a fuel injector includes a guide hole and a spill chamber both of which are formed in a body of the fuel injector and communicate with each other. A valve seat is formed on one end surface of the guide hole facing the spill chamber. The electromagnetic valve mechanism also includes a poppet-type valve member which has a stem portion and a head (abutment portion) formed at one end of the stem portion. The stem portion has a reduced diameter portion disposed adjacent to the head, and is slidably received in the guide hole, so that an annular space is formed between the reduced diameter portion and the inner peripheral surface of the guide hole. This annular space is in communication with a pump chamber. The head of the valve member is disposed within the spill chamber and is movable into and out of contact with the valve seat. The electromagnetic valve mechanism further includes a solenoid drive means for controlling the movement of the valve member. The solenoid drive means comprises an armature connected to the other end of the stem portion of the valve member, a solenoid for driving the armature to move the same toward the valve seat together with the valve member so that the head of the valve member abuts against the valve seat, and a spring for urging the valve member in a direction away from the valve seat. The armature is mounted within an armature chamber formed in the fuel injector body.
In the above conventional unit fuel injector, when the solenoid is energized during the pump stroke of the plunger, the head of the valve member is brought into engagement with the valve seat, so that the communication of the spill chamber with the pump chamber is interrupted. As a result, the fuel within the pump chamber is pressurized and is injected from an injection nozzle mechanism. At this time, the fuel pressure is applied to the opposed shoulders formed respectively on the opposite ends of the reduced diameter portion of the valve member, so that the forces acting respectively on these opposed shoulders cancel each other. Therefore, the force required to be produced by the solenoid so as to hold the valve member in its closed position can be relatively small. When the solenoid is de-energized during the pump stroke of the plunger, the head of the valve member is brought out of engagement with the valve seat under the influence of the spring, so that the fuel of high pressure within the pump chamber is spilled to the spill chamber. As a result, the pressure within the pump chamber decreases, thus terminating the fuel injection operation.
At the moment when the valve member is disengaged from the valve seat so as to terminate the fuel injection operation as described above, the pressure within the spill chamber becomes high, and the valve member is urged by this high pressure toward the valve seat. As a result, the speed of disengagement of the valve member from the valve seat under the influence of the spring may become slower, or the valve member may be instantaneously moved back toward the valve seat. This results in a problem that the area of flow between the valve seat and the valve member can not be increased quickly, so that the pressure drop in the pump chamber is retarded. As a result, the fuel injection operation fails to be completely terminated at a time, and the problem of subsequent dripping of the fuel is encountered.
In order to overcome this problem, the above-mentioned U. S. patents have proposed the following procedure. Specifically, the spill chamber is communicated with the armature chamber by a passage, formed in the valve member, and/or a passage formed in the body. With this arrangement, the pressure of the above-mentioned spill fuel is applied not only to the spill chamber but also to the armature chamber, so that substantially the same pressure acts on both of the opposite ends of the valve member, thereby canceling the forces acting on the valve member in opposite axial directions of the valve member.
A fuel injector disclosed in U.S. Pat. No. 4,470,545 is provided with means by which the spill fuel pressure serves to accelerate the movement of a valve member away from a valve seat when the valve member is moved in its opening direction. More specifically, the valve member has a second stem portion extending from its head away from the valve seat, and the second stem portion has a flange intermediate opposite ends thereof. A slide hole is formed in the body of the fuel injector adjacent to a spill chamber, and an auxiliary chamber is formed in the body adjacent to one end of the slide hole remote from the spill chamber. The slide hole is smaller in diameter than the spill chamber and the auxiliary chamber. The flange of the valve member is slidably received in the slide hole. A passage is formed in the valve member, and has a first portion communicating the spill chamber with the armature chamber to achieve the above-mentioned pressure balance, and a second portion communicating the auxiliary chamber with the spill chamber and cooperating with the first portion to communicate the auxiliary chamber with the armature chamber.
With this conventional construction, immediately when the head of the valve member is disengaged from the valve seat, kinetic energy of the spill fuel flowing into the spill chamber is applied to the flange of the valve member so as to accelerate the movement of the valve member away from the valve seat. Actually, however, the amount of such spill fuel is small, and the kinetic energy obtained is also small, and therefore such acceleration effect has been found not satisfactory. Further, the spill fuel pressure is applied to one face or side of the flange facing the spill chamber when the valve member begins to move in its opening direction, and substantially simultaneously therewith, the spill fuel pressure is also transmitted to the other side of the flange facing the auxiliary chamber via the passage formed in the valve member. Therefore, the force produced by the spill fuel pressure so as to propel or move the valve member away from the valve seat is hardly effective.